The present invention relates to an improved molten metal sample cup for measuring solidifying temperatures of cast iron, steel and similar materials. A molten metal receiving cup, usually made from suitable refractory materials, such as foundry sand and cement, is fitted with a thermocouple for determining temperature, phase changes of the molten metal, thermal arrest temperatures and the like. For example, see the cups of U.S. Pat. No. Re. 26,409.
In present molten metal receiving cups, the thermocouple hot junction may be supported within the cup in any number of ways so as to contact the molten metal when the metal is poured into the cup. The thermocouple is able to sense the temperature of the molten metal as it cools, and a temperature versus time chart is plotted. By interpreting the chart, metallurgists can determine the characteristics of the molten metal.
Usually, the samplers are provided with an inside coating to insulate the thermocouple and/or to influence the solidification of the molten metal. When cast iron cools, it solidifies into a combination of gray and white iron. Gray iron refers to iron having enhanced graphite formation. White iron refers to iron having enhanced carbide formation, and the process of forming white iron is called white solidification.
The molten metal sample cups are frequently coated with a material which promotes the formation of either gray iron or white iron. When white solidification is desired, a material which enhances the formation of carbides is added to the coating on the inside of the sample cup. Typical additive substances for obtaining white solidification are tellurium, bismuth, antimony and materials which evolve hydrogen, such as soluble glass. Their use, however, results in problems because they have a melting point which is lower than that of cast iron or steel, and a boiling point which lies below or within the temperature range of the molten metal on which the measurements are to be taken.
Due to these problems, it has been proposed to use refractory materials as additives in the coating to provide a slight protection of the carbide forming substances, so as to retard the mixing of these substances with the molten metal. The trend has been toward applying these coatings as uniformly as possible on the entire interior surface of the sample cups, probably to achieve a good dispersion of the additives in the metal. The application of these coatings, however, is a time-consuming and costly operation, especially considering the fact that the sample cups may be used only once.
Other problems exist by coating the entire interior surface of the sample cups. Because of the evolution of gases from the coating when the metal is poured into the cup, it has been impossible to fill the cup with metal in one step. Two-step filling of the cup has a detrimental influence on the measurement of the solidifying temperatures. In coating the interior of the cups, the coating was applied after the thermocouples have been placed in them, for economy reasons (otherwise it would have been necessary either to use plugs or to rebore the holes for the thermocouple). This results in an extra coating being applied on the thermocouples. Especially in the case of centrally or axially located thermocouples, when the metal was poured into the cup and gases were evolved from the coating, a slot-like air or gas pocket could be created, resulting in the measurement being inaccurate or even false. In addition, the coating absorbed a certain degree of heat which could effect the accuracy of the measurement.
U.S. Pat. No. 3,546,921, issued to Bourke et al., discloses a method of producing an initial thermal arrest in the cooling curve of a molten sample of hypereutectic cast iron by introducing into the sample a stabilizing additive which retards primary graphite formation as the molten sample cools. The stabilizer is added in the form of a pellet or in particulate form immediately after the sample of molten metal is poured. Alternatively, the stabilizer can be added to the sampling device prior to the introduction of the molten sample. The stabilizing additive includes at least one member of the group consisting of bismuth, boron, cerium, lead, magnesium and tellurium.
The tellurium used in Bourke et al. is not mixed with refractory materials to slow the mixing of the tellurium with the molten metal. As a result, the tellurium pellet tends to become burned up and not available for white solidification. The efficiency of the tellurium in promoting white solidification is greatly dependent on the pouring temperature of the molten metal. If the pouring temperature is too high, a part of the tellurium will burn, resulting in only partial white solidification. If the pouring temperature is too low, the sample will solidify before becoming thoroughly mixed with the tellurium, again resulting in partial white solidification.
Another problem inherent in the use of carbide stabilizers in pellet form or particulate form is the tendency for the stabilizer to rise to the surface of the molten metal as a slag. This is particularly true with higher carbon equivalent irons. When this occurs, the poured iron is not completely chilled so as to obtain a white sample.
In the foundry industry, there is a commercially used mold dressing, purportedly in accordance with U.S. Pat. No. 3,275,460, issued to Jeanneret, which I believe is the closest material to the blob used in this invention.